The invention is directed to a process for extruding and granulating thermoplastic material. A conventional process for extruding and granulating or pelletizing plastic material comprises extruding a thermoplastic through an extrusion die including a die plate which on its face is provided with an insulation member, the die plate being provided with extrusion holes for the extrusion therethrough of thermoplastic material. If granulation is desired, a rotatably mounted knife is provided adjacent to the face of the extrusion die for cutting the material extruded through the extrusion holes into granules. The thermoplastic material is supplied to the extrusion die by an extruder, usually of the worm screw type, which presses the material in heated and molten condition through the extrusion holes. The die plate may be heated by steam, hot oil or electricity for maintaining the passing thermoplastic material at the required extrusion temperature. If it is desired to pelletize the material, the extruded material, on leaving the extrusion holes in the form of strands, is cut into small particles by a knife sweeping over the face of the die, and the extrudate is cooled to its solidification temperature by contact with a cooling liquid.
The cooling liquid, normally water, is supplied by spraying or pouring it over the face of the die, or the die is positioned with its face in contact with water contained in a water chamber. To avoid undue cooling of the hot die by the cooling water, the die plate may be thermally insulated by the provision of insulating material on its face. A common material used for this purpose is ceramic material, but this and various other insulation materials are not always very satisfactory for a number of reasons. Ceramic material is relatively weak and fragile, and therefore vulnerable. When the die has been in operation for some length of time, parts of the layer of ceramic material may have broken out, and repair or replacement of the layer becomes necessary.
If the insulation layer has become damaged, the extrusion holes are not uniformly heated, and poorly insulated extrusion holes enhance the risk of "freezing" (solidification) of the thermoplastic material in the holes during operation.
This invention is a process and apparatus for extruding and granulating thermoplastics which gives excellent insulation and overcomes the problems associated with the use of face-cut pelletizers having conventional insulated dies by the use of an improved insulation member for the die plate.